ELF(5)                                               Linux Programmer's Manual                                              ELF(5)



NAME
       elf - format of Executable and Linking Format (ELF) files

SYNOPSIS
       #include <elf.h>

DESCRIPTION
       The  header  file  <elf.h>  defines  the  format of ELF executable binary files.  Amongst these files are normal executable
       files, relocatable object files, core files and shared libraries.

       An executable file using the ELF file format consists of an ELF header, followed by a program header  table  or  a  section
       header  table,  or  both.   The  ELF header is always at offset zero of the file.  The program header table and the section
       header table's offset in the file are defined in the ELF header.  The two tables describe the rest of  the  particularities
       of the file.

       This  header  file describes the above mentioned headers as C structures and also includes structures for dynamic sections,
       relocation sections and symbol tables.

       The following types are used for N-bit architectures (N=32,64, ElfN stands for Elf32 or Elf64, uintN_t stands for  uint32_t
       or uint64_t):

           ElfN_Addr       Unsigned program address, uintN_t
           ElfN_Off        Unsigned file offset, uintN_t
           ElfN_Section    Unsigned section index, uint16_t
           ElfN_Versym     Unsigned version symbol information, uint16_t
           Elf_Byte        unsigned char
           ElfN_Half       uint16_t
           ElfN_Sword      int32_t
           ElfN_Word       uint32_t
           ElfN_Sxword     int64_t
           ElfN_Xword      uint64_t

       (Note: The *BSD terminology is a bit different.  There Elf64_Half is twice as large as Elf32_Half, and Elf64Quarter is used
       for uint16_t.  In order to avoid confusion these types are replaced by explicit ones in the below.)

       All data structures that the file format defines follow the "natural" size and alignment guidelines for the relevant class.
       If  necessary,  data  structures contain explicit padding to ensure 4-byte alignment for 4-byte objects, to force structure
       sizes to a multiple of 4, etc.

       The ELF header is described by the type Elf32_Ehdr or Elf64_Ehdr:

           #define EI_NIDENT 16

           typedef struct {
               unsigned char e_ident[EI_NIDENT];
               uint16_t      e_type;
               uint16_t      e_machine;
               uint32_t      e_version;
               ElfN_Addr     e_entry;
               ElfN_Off      e_phoff;
               ElfN_Off      e_shoff;
               uint32_t      e_flags;
               uint16_t      e_ehsize;
               uint16_t      e_phentsize;
               uint16_t      e_phnum;
               uint16_t      e_shentsize;
               uint16_t      e_shnum;
               uint16_t      e_shstrndx;
           } ElfN_Ehdr;

       The fields have the following meanings:

       e_ident     This array of bytes specifies to interpret the file, independent of the processor or the file's remaining  con‐
                   tents.  Within this array everything is named by macros, which start with the prefix EI_ and may contain values
                   which start with the prefix ELF.  The following macros are defined:

                   EI_MAG0     The first byte of the magic number.  It must be filled with ELFMAG0.  (0: 0x7f)

                   EI_MAG1     The second byte of the magic number.  It must be filled with ELFMAG1.  (1: 'E')

                   EI_MAG2     The third byte of the magic number.  It must be filled with ELFMAG2.  (2: 'L')

                   EI_MAG3     The fourth byte of the magic number.  It must be filled with ELFMAG3.  (3: 'F')

                   EI_CLASS    The fifth byte identifies the architecture for this binary:

                               ELFCLASSNONE  This class is invalid.
                               ELFCLASS32    This defines the 32-bit architecture.  It supports machines with  files  and  virtual
                                             address spaces up to 4 Gigabytes.
                               ELFCLASS64    This defines the 64-bit architecture.

                   EI_DATA     The  sixth  byte specifies the data encoding of the processor-specific data in the file.  Currently
                               these encodings are supported:

                               ELFDATANONE   Unknown data format.
                               ELFDATA2LSB   Two's complement, little-endian.
                               ELFDATA2MSB   Two's complement, big-endian.

                   EI_VERSION  The seventh byte is the version number of the ELF specification:
                               EV_NONE       Invalid version.
                               EV_CURRENT    Current version.

                   EI_OSABI    The eighth byte identifies the operating system and ABI to which  the  object  is  targeted.   Some
                               fields  in  other  ELF  structures  have flags and values that have platform-specific meanings; the
                               interpretation of those fields is determined by the value of this byte.  E.g.:

                               ELFOSABI_NONE       Same as ELFOSABI_SYSV
                               ELFOSABI_SYSV       UNIX System V ABI.
                               ELFOSABI_HPUX       HP-UX ABI.
                               ELFOSABI_NETBSD     NetBSD ABI.
                               ELFOSABI_LINUX      Linux ABI.
                               ELFOSABI_SOLARIS    Solaris ABI.
                               ELFOSABI_IRIX       IRIX ABI.
                               ELFOSABI_FREEBSD    FreeBSD ABI.
                               ELFOSABI_TRU64      TRU64 UNIX ABI.
                               ELFOSABI_ARM        ARM architecture ABI.
                               ELFOSABI_STANDALONE Stand-alone (embedded) ABI.

                   EI_ABIVERSION
                               The ninth byte identifies the version of the ABI to which the object is targeted.   This  field  is
                               used to distinguish among incompatible versions of an ABI.  The interpretation of this version num‐
                               ber is dependent on the ABI identified by the EI_OSABI  field.   Applications  conforming  to  this
                               specification use the value 0.

                   EI_PAD      Start  of  padding.   These  bytes  are  reserved and set to zero.  Programs which read them should
                               ignore them.  The value for EI_PAD will change in the future if currently unused  bytes  are  given
                               meanings.

                   EI_NIDENT   The size of the e_ident array.

       e_type      This member of the structure identifies the object file type:

                   ET_NONE     An unknown type.
                   ET_REL      A relocatable file.
                   ET_EXEC     An executable file.
                   ET_DYN      A shared object.
                   ET_CORE     A core file.

       e_machine   This member specifies the required architecture for an individual file.  E.g.:

                   EM_NONE     An unknown machine.
                   EM_M32      AT&T WE 32100.
                   EM_SPARC    Sun Microsystems SPARC.
                   EM_386      Intel 80386.
                   EM_68K      Motorola 68000.
                   EM_88K      Motorola 88000.
                   EM_860      Intel 80860.
                   EM_MIPS     MIPS RS3000 (big-endian only).
                   EM_PARISC   HP/PA.
                   EM_SPARC32PLUS
                               SPARC with enhanced instruction set.
                   EM_PPC      PowerPC.
                   EM_PPC64    PowerPC 64-bit.
                   EM_S390     IBM S/390
                   EM_ARM      Advanced RISC Machines
                   EM_SH       Renesas SuperH
                   EM_SPARCV9  SPARC v9 64-bit.
                   EM_IA_64    Intel Itanium
                   EM_X86_64   AMD x86-64
                   EM_VAX      DEC Vax.

       e_version   This member identifies the file version:

                   EV_NONE     Invalid version.
                   EV_CURRENT  Current version.

       e_entry     This  member  gives the virtual address to which the system first transfers control, thus starting the process.
                   If the file has no associated entry point, this member holds zero.

       e_phoff     This member holds the program header table's file offset in bytes.  If the file has no  program  header  table,
                   this member holds zero.

       e_shoff     This  member  holds  the  section header table's file offset in bytes.  If the file has no section header table
                   this member holds zero.

       e_flags     This  member  holds  processor-specific  flags  associated  with  the  file.   Flag   names   take   the   form
                   EF_`machine_flag'.  Currently no flags have been defined.

       e_ehsize    This member holds the ELF header's size in bytes.

       e_phentsize This  member  holds the size in bytes of one entry in the file's program header table; all entries are the same
                   size.

       e_phnum     This member holds the number of entries in the program header table.   Thus  the  product  of  e_phentsize  and
                   e_phnum gives the table's size in bytes.  If a file has no program header, e_phnum holds the value zero.

                   If  the  number of entries in the program header table is larger than or equal to PN_XNUM (0xffff), this member
                   holds PN_XNUM (0xffff) and the real number of entries in the program header table is held in the sh_info member
                   of  the initial entry in section header table.  Otherwise, the sh_info member of the initial entry contains the
                   value zero.

                   PN_XNUM  This is defined as 0xffff, the largest number e_phnum can have, specifying where the actual number  of
                            program headers is assigned.

       e_shentsize This  member  holds a sections header's size in bytes.  A section header is one entry in the section header ta‐
                   ble; all entries are the same size.

       e_shnum     This member holds the number of entries in the section header table.   Thus  the  product  of  e_shentsize  and
                   e_shnum  gives  the section header table's size in bytes.  If a file has no section header table, e_shnum holds
                   the value of zero.

                   If the number of entries in the section header table is larger than or equal to SHN_LORESERVE (0xff00), e_shnum
                   holds  the  value zero and the real number of entries in the section header table is held in the sh_size member
                   of the initial entry in section header table.  Otherwise, the sh_size member of the initial entry in  the  sec‐
                   tion header table holds the value zero.

       e_shstrndx  This  member  holds  the section header table index of the entry associated with the section name string table.
                   If the file has no section name string table, this member holds the value SHN_UNDEF.

                   If the index of section name string table section is larger than or equal to SHN_LORESERVE (0xff00), this  mem‐
                   ber  holds  SHN_XINDEX  (0xffff)  and  the  real  index of the section name string table section is held in the
                   sh_link member of the initial entry in section header table.  Otherwise, the  sh_link  member  of  the  initial
                   entry in section header table contains the value zero.

                   SHN_UNDEF     This  value  marks an undefined, missing, irrelevant, or otherwise meaningless section reference.
                                 For example, a symbol "defined" relative to section number SHN_UNDEF is an undefined symbol.

                   SHN_LORESERVE This value specifies the lower bound of the range of reserved indices.

                   SHN_LOPROC    Values greater than or equal to SHN_HIPROC are reserved for processor-specific semantics.

                   SHN_HIPROC    Values less than or equal to SHN_LOPROC are reserved for processor-specific semantics.

                   SHN_ABS       This value specifies absolute values for  the  corresponding  reference.   For  example,  symbols
                                 defined  relative  to section number SHN_ABS have absolute values and are not affected by reloca‐
                                 tion.

                   SHN_COMMON    Symbols defined relative to this section are common symbols, such as Fortran  COMMON  or  unallo‐
                                 cated C external variables.

                   SHN_HIRESERVE This  value  specifies the upper bound of the range of reserved indices between SHN_LORESERVE and
                                 SHN_HIRESERVE, inclusive; the values do not reference the section header  table.   That  is,  the
                                 section header table does not contain entries for the reserved indices.

       An  executable  or  shared object file's program header table is an array of structures, each describing a segment or other
       information the system needs to prepare the program for execution.  An object file segment contains one or  more  sections.
       Program  headers  are meaningful only for executable and shared object files.  A file specifies its own program header size
       with the ELF header's e_phentsize and e_phnum members.  The ELF program header is  described  by  the  type  Elf32_Phdr  or
       Elf64_Phdr depending on the architecture:

           typedef struct {
               uint32_t   p_type;
               Elf32_Off  p_offset;
               Elf32_Addr p_vaddr;
               Elf32_Addr p_paddr;
               uint32_t   p_filesz;
               uint32_t   p_memsz;
               uint32_t   p_flags;
               uint32_t   p_align;
           } Elf32_Phdr;

           typedef struct {
               uint32_t   p_type;
               uint32_t   p_flags;
               Elf64_Off  p_offset;
               Elf64_Addr p_vaddr;
               Elf64_Addr p_paddr;
               uint64_t   p_filesz;
               uint64_t   p_memsz;
               uint64_t   p_align;
           } Elf64_Phdr;

       The  main  difference  between  the  32-bit and the 64-bit program header lies in the location of the p_flags member in the
       total struct.

       p_type      This member of the Phdr struct tells what kind of segment this array element describes or how to interpret  the
                   array element's information.

                   PT_NULL     The  array  element  is  unused and the other members' values are undefined.  This lets the program
                               header have ignored entries.

                   PT_LOAD     The array element specifies a loadable segment, described by p_filesz and p_memsz.  The bytes  from
                               the  file  are mapped to the beginning of the memory segment.  If the segment's memory size p_memsz
                               is larger than the file size p_filesz, the "extra" bytes are defined to hold the  value  0  and  to
                               follow  the  segment's  initialized  area.   The  file size may not be larger than the memory size.
                               Loadable segment entries in the program header table appear  in  ascending  order,  sorted  on  the
                               p_vaddr member.

                   PT_DYNAMIC  The array element specifies dynamic linking information.

                   PT_INTERP   The  array  element  specifies  the location and size of a null-terminated pathname to invoke as an
                               interpreter.  This segment type is meaningful only for executable files (though it  may  occur  for
                               shared  objects).   However  it  may not occur more than once in a file.  If it is present, it must
                               precede any loadable segment entry.

                   PT_NOTE     The array element specifies the location and size for auxiliary information.

                   PT_SHLIB    This segment type is reserved but has unspecified semantics.  Programs that contain an  array  ele‐
                               ment of this type do not conform to the ABI.

                   PT_PHDR     The  array element, if present, specifies the location and size of the program header table itself,
                               both in the file and in the memory image of the program.  This segment type may not occur more than
                               once  in  a  file.   Moreover,  it may occur only if the program header table is part of the memory
                               image of the program.  If it is present, it must precede any loadable segment entry.

                   PT_LOPROC   Values greater than or equal to PT_HIPROC are reserved for processor-specific semantics.

                   PT_HIPROC   Values less than or equal to PT_LOPROC are reserved for processor-specific semantics.

                   PT_GNU_STACK
                               GNU extension which is used by the Linux kernel to control the state of the stack via the flags set
                               in the p_flags member.

       p_offset    This member holds the offset from the beginning of the file at which the first byte of the segment resides.

       p_vaddr     This member holds the virtual address at which the first byte of the segment resides in memory.

       p_paddr     On  systems  for  which  physical  addressing  is  relevant, this member is reserved for the segment's physical
                   address.  Under BSD this member is not used and must be zero.

       p_filesz    This member holds the number of bytes in the file image of the segment.  It may be zero.

       p_memsz     This member holds the number of bytes in the memory image of the segment.  It may be zero.

       p_flags     This member holds a bit mask of flags relevant to the segment:

                   PF_X   An executable segment.
                   PF_W   A writable segment.
                   PF_R   A readable segment.

                   A text segment commonly has the flags PF_X and PF_R.  A data segment commonly has PF_X, PF_W and PF_R.

       p_align     This member holds the value to which the segments are aligned in memory and in the file.  Loadable process seg‐
                   ments  must  have congruent values for p_vaddr and p_offset, modulo the page size.  Values of zero and one mean
                   no alignment is required.  Otherwise, p_align should be a positive, integral power of two, and  p_vaddr  should
                   equal p_offset, modulo p_align.

       A  file's section header table lets one locate all the file's sections.  The section header table is an array of Elf32_Shdr
       or Elf64_Shdr structures.  The ELF header's e_shoff member gives the byte offset from the beginning of the file to the sec‐
       tion  header  table.  e_shnum holds the number of entries the section header table contains.  e_shentsize holds the size in
       bytes of each entry.

       A section header table index is a subscript into this array.  Some section header table indices are reserved:  the  initial
       entry  and  the  indices between SHN_LORESERVE and SHN_HIRESERVE.  The initial entry is used in ELF extensions for e_phnum,
       e_shnum and e_strndx; in other cases, each field in the initial entry is set to zero.  An object file does  not  have  sec‐
       tions for these special indices:

              SHN_UNDEF     This value marks an undefined, missing, irrelevant or otherwise meaningless section reference.

              SHN_LORESERVE This value specifies the lower bound of the range of reserved indices.

              SHN_LOPROC    Values greater than or equal to SHN_HIPROC are reserved for processor-specific semantics.

              SHN_HIPROC    Values less than or equal to SHN_LOPROC are reserved for processor-specific semantics.

              SHN_ABS       This  value  specifies  the  absolute  value  for  the corresponding reference.  For example, a symbol
                            defined relative to section number SHN_ABS has an absolute value and is not affected by relocation.

              SHN_COMMON    Symbols defined relative to this section are common symbols, such as FORTRAN COMMON or  unallocated  C
                            external variables.

              SHN_HIRESERVE This  value  specifies  the upper bound of the range of reserved indices.  The system reserves indices
                            between SHN_LORESERVE and SHN_HIRESERVE, inclusive.  The section header table does not contain entries
                            for the reserved indices.

       The section header has the following structure:

           typedef struct {
               uint32_t   sh_name;
               uint32_t   sh_type;
               uint32_t   sh_flags;
               Elf32_Addr sh_addr;
               Elf32_Off  sh_offset;
               uint32_t   sh_size;
               uint32_t   sh_link;
               uint32_t   sh_info;
               uint32_t   sh_addralign;
               uint32_t   sh_entsize;
           } Elf32_Shdr;

           typedef struct {
               uint32_t   sh_name;
               uint32_t   sh_type;
               uint64_t   sh_flags;
               Elf64_Addr sh_addr;
               Elf64_Off  sh_offset;
               uint64_t   sh_size;
               uint32_t   sh_link;
               uint32_t   sh_info;
               uint64_t   sh_addralign;
               uint64_t   sh_entsize;
           } Elf64_Shdr;

       No real differences exist between the 32-bit and 64-bit section headers.

       sh_name   This  member  specifies the name of the section.  Its value is an index into the section header string table sec‐
                 tion, giving the location of a null-terminated string.

       sh_type   This member categorizes the section's contents and semantics.

                 SHT_NULL       This value marks the section header as inactive.  It does not have an associated  section.   Other
                                members of the section header have undefined values.

                 SHT_PROGBITS   This  section  holds  information  defined by the program, whose format and meaning are determined
                                solely by the program.

                 SHT_SYMTAB     This section holds a symbol table.  Typically,  SHT_SYMTAB  provides  symbols  for  link  editing,
                                though  it  may also be used for dynamic linking.  As a complete symbol table, it may contain many
                                symbols unnecessary for dynamic linking.  An object file can also contain a SHT_DYNSYM section.

                 SHT_STRTAB     This section holds a string table.  An object file may have multiple string table sections.

                 SHT_RELA       This section holds relocation entries with explicit addends,  such  as  type  Elf32_Rela  for  the
                                32-bit class of object files.  An object may have multiple relocation sections.

                 SHT_HASH       This section holds a symbol hash table.  An object participating in dynamic linking must contain a
                                symbol hash table.  An object file may have only one hash table.

                 SHT_DYNAMIC    This section holds information for dynamic linking.  An object file may have only one dynamic sec‐
                                tion.

                 SHT_NOTE       This section holds information that marks the file in some way.

                 SHT_NOBITS     A  section  of  this  type  occupies  no  space  in the file but otherwise resembles SHT_PROGBITS.
                                Although this section contains no bytes, the sh_offset member contains the conceptual file offset.

                 SHT_REL        This section holds relocation offsets without explicit addends, such as  type  Elf32_Rel  for  the
                                32-bit class of object files.  An object file may have multiple relocation sections.

                 SHT_SHLIB      This section is reserved but has unspecified semantics.

                 SHT_DYNSYM     This  section  holds  a minimal set of dynamic linking symbols.  An object file can also contain a
                                SHT_SYMTAB section.

                 SHT_LOPROC     This value up to and including SHT_HIPROC is reserved for processor-specific semantics.

                 SHT_HIPROC     This value down to and including SHT_LOPROC is reserved for processor-specific semantics.

                 SHT_LOUSER     This value specifies the lower bound of the range of indices reserved for application programs.

                 SHT_HIUSER     This value specifies the upper bound of the range of indices reserved  for  application  programs.
                                Section  types between SHT_LOUSER and SHT_HIUSER may be used by the application, without conflict‐
                                ing with current or future system-defined section types.

       sh_flags  Sections support one-bit flags that describe miscellaneous attributes.  If a flag bit is  set  in  sh_flags,  the
                 attribute  is  "on"  for the section.  Otherwise, the attribute is "off" or does not apply.  Undefined attributes
                 are set to zero.

                 SHF_WRITE      This section contains data that should be writable during process execution.

                 SHF_ALLOC      This section occupies memory during process execution.  Some control sections do not reside in the
                                memory image of an object file.  This attribute is off for those sections.

                 SHF_EXECINSTR  This section contains executable machine instructions.

                 SHF_MASKPROC   All bits included in this mask are reserved for processor-specific semantics.

       sh_addr   If  this  section  appears in the memory image of a process, this member holds the address at which the section's
                 first byte should reside.  Otherwise, the member contains zero.

       sh_offset This member's value holds the byte offset from the beginning of the file to the first byte in the  section.   One
                 section  type,  SHT_NOBITS, occupies no space in the file, and its sh_offset member locates the conceptual place‐
                 ment in the file.

       sh_size   This member holds the section's size in bytes.  Unless the section  type  is  SHT_NOBITS,  the  section  occupies
                 sh_size bytes in the file.  A section of type SHT_NOBITS may have a nonzero size, but it occupies no space in the
                 file.

       sh_link   This member holds a section header table index link, whose interpretation depends on the section type.

       sh_info   This member holds extra information, whose interpretation depends on the section type.

       sh_addralign
                 Some sections have address alignment constraints.  If a section holds a doubleword, the system must  ensure  dou‐
                 bleword  alignment  for  the entire section.  That is, the value of sh_addr must be congruent to zero, modulo the
                 value of sh_addralign.  Only zero and positive integral powers of two are allowed.  Values of zero  or  one  mean
                 the section has no alignment constraints.

       sh_entsize
                 Some sections hold a table of fixed-sized entries, such as a symbol table.  For such a section, this member gives
                 the size in bytes for each entry.  This member contains zero if the section does not hold a table  of  fixed-size
                 entries.

       Various sections hold program and control information:

       .bss      This  section holds uninitialized data that contributes to the program's memory image.  By definition, the system
                 initializes the data with zeros when the program begins to  run.   This  section  is  of  type  SHT_NOBITS.   The
                 attribute types are SHF_ALLOC and SHF_WRITE.

       .comment  This  section  holds  version control information.  This section is of type SHT_PROGBITS.  No attribute types are
                 used.

       .ctors    This section holds initialized pointers to the C++ constructor functions.  This section is of type  SHT_PROGBITS.
                 The attribute types are SHF_ALLOC and SHF_WRITE.

       .data     This  section  holds  initialized  data  that  contribute to the program's memory image.  This section is of type
                 SHT_PROGBITS.  The attribute types are SHF_ALLOC and SHF_WRITE.

       .data1    This section holds initialized data that contribute to the program's memory  image.   This  section  is  of  type
                 SHT_PROGBITS.  The attribute types are SHF_ALLOC and SHF_WRITE.

       .debug    This  section  holds  information for symbolic debugging.  The contents are unspecified.  This section is of type
                 SHT_PROGBITS.  No attribute types are used.

       .dtors    This section holds initialized pointers to the C++ destructor functions.  This section is of  type  SHT_PROGBITS.
                 The attribute types are SHF_ALLOC and SHF_WRITE.

       .dynamic  This  section  holds  dynamic  linking  information.   The  section's  attributes will include the SHF_ALLOC bit.
                 Whether the SHF_WRITE bit is set is processor-specific.  This section is of type SHT_DYNAMIC.  See the attributes
                 above.

       .dynstr   This section holds strings needed for dynamic linking, most commonly the strings that represent the names associ‐
                 ated with symbol table entries.  This section is of type SHT_STRTAB.  The attribute type used is SHF_ALLOC.

       .dynsym   This section holds the dynamic linking symbol table.  This section is of type SHT_DYNSYM.  The attribute used  is
                 SHF_ALLOC.

       .fini     This section holds executable instructions that contribute to the process termination code.  When a program exits
                 normally the system arranges to execute the code in this section.  This section is  of  type  SHT_PROGBITS.   The
                 attributes used are SHF_ALLOC and SHF_EXECINSTR.

       .gnu.version
                 This  section  holds  the  version  symbol  table,  an  array  of  ElfN_Half  elements.   This section is of type
                 SHT_GNU_versym.  The attribute type used is SHF_ALLOC.

       .gnu.version_d
                 This section holds the version symbol definitions, a table of ElfN_Verdef structures.  This section  is  of  type
                 SHT_GNU_verdef.  The attribute type used is SHF_ALLOC.

       .gnu.version_r
                 This  section  holds  the version symbol needed elements, a table of ElfN_Verneed structures.  This section is of
                 type SHT_GNU_versym.  The attribute type used is SHF_ALLOC.

       .got      This section holds the global offset table.  This section is of type SHT_PROGBITS.  The attributes are  processor
                 specific.

       .hash     This section holds a symbol hash table.  This section is of type SHT_HASH.  The attribute used is SHF_ALLOC.

       .init     This  section  holds  executable instructions that contribute to the process initialization code.  When a program
                 starts to run the system arranges to execute the code in this section  before  calling  the  main  program  entry
                 point.  This section is of type SHT_PROGBITS.  The attributes used are SHF_ALLOC and SHF_EXECINSTR.

       .interp   This  section  holds the pathname of a program interpreter.  If the file has a loadable segment that includes the
                 section, the section's attributes will include the SHF_ALLOC bit.  Otherwise, that bit will be off.  This section
                 is of type SHT_PROGBITS.

       .line     This section holds line number information for symbolic debugging, which describes the correspondence between the
                 program source and the machine code.  The contents are unspecified.  This section is of  type  SHT_PROGBITS.   No
                 attribute types are used.

       .note     This  section  holds  information  in the "Note Section" format.  This section is of type SHT_NOTE.  No attribute
                 types are used.  OpenBSD native executables usually contain a .note.openbsd.ident section to identify themselves,
                 for the kernel to bypass any compatibility ELF binary emulation tests when loading the file.

       .note.GNU-stack
                 This section is used in Linux object files for declaring stack attributes.  This section is of type SHT_PROGBITS.
                 The only attribute used is SHF_EXECINSTR.  This indicates to the GNU linker that the object file requires an exe‐
                 cutable stack.

       .plt      This  section  holds the procedure linkage table.  This section is of type SHT_PROGBITS.  The attributes are pro‐
                 cessor specific.

       .relNAME  This section holds relocation information as described below.  If the file has a loadable segment  that  includes
                 relocation,  the section's attributes will include the SHF_ALLOC bit.  Otherwise the bit will be off.  By conven‐
                 tion, "NAME" is supplied by the section to which the relocations apply.  Thus a relocation section for .text nor‐
                 mally would have the name .rel.text.  This section is of type SHT_REL.

       .relaNAME This  section  holds relocation information as described below.  If the file has a loadable segment that includes
                 relocation, the section's attributes will include the SHF_ALLOC bit.  Otherwise the bit will be off.  By  conven‐
                 tion, "NAME" is supplied by the section to which the relocations apply.  Thus a relocation section for .text nor‐
                 mally would have the name .rela.text.  This section is of type SHT_RELA.

       .rodata   This section holds read-only data that typically contributes to a nonwritable segment in the process image.  This
                 section is of type SHT_PROGBITS.  The attribute used is SHF_ALLOC.

       .rodata1  This section holds read-only data that typically contributes to a nonwritable segment in the process image.  This
                 section is of type SHT_PROGBITS.  The attribute used is SHF_ALLOC.

       .shstrtab This section holds section names.  This section is of type SHT_STRTAB.  No attribute types are used.

       .strtab   This section holds strings, most commonly the strings that represent  the  names  associated  with  symbol  table
                 entries.  If the file has a loadable segment that includes the symbol string table, the section's attributes will
                 include the SHF_ALLOC bit.  Otherwise the bit will be off.  This section is of type SHT_STRTAB.

       .symtab   This section holds a symbol table.  If the file has a loadable segment that includes the symbol table,  the  sec‐
                 tion's  attributes  will  include  the  SHF_ALLOC  bit.   Otherwise the bit will be off.  This section is of type
                 SHT_SYMTAB.

       .text     This section holds the "text", or executable instructions, of a program.  This section is of  type  SHT_PROGBITS.
                 The attributes used are SHF_ALLOC and SHF_EXECINSTR.

       String  table  sections  hold  null-terminated  character  sequences,  commonly called strings.  The object file uses these
       strings to represent symbol and section names.  One references a string as an index into the  string  table  section.   The
       first  byte,  which is index zero, is defined to hold a null byte ('\0').  Similarly, a string table's last byte is defined
       to hold a null byte, ensuring null termination for all strings.

       An object file's symbol table holds information needed to locate and relocate a program's symbolic definitions  and  refer‐
       ences.  A symbol table index is a subscript into this array.

           typedef struct {
               uint32_t      st_name;
               Elf32_Addr    st_value;
               uint32_t      st_size;
               unsigned char st_info;
               unsigned char st_other;
               uint16_t      st_shndx;
           } Elf32_Sym;

           typedef struct {
               uint32_t      st_name;
               unsigned char st_info;
               unsigned char st_other;
               uint16_t      st_shndx;
               Elf64_Addr    st_value;
               uint64_t      st_size;
           } Elf64_Sym;

       The 32-bit and 64-bit versions have the same members, just in a different order.

       st_name   This  member  holds an index into the object file's symbol string table, which holds character representations of
                 the symbol names.  If the value is nonzero, it represents a string table index that gives the symbol name.   Oth‐
                 erwise, the symbol table has no name.

       st_value  This member gives the value of the associated symbol.

       st_size   Many symbols have associated sizes.  This member holds zero if the symbol has no size or an unknown size.

       st_info   This member specifies the symbol's type and binding attributes:

                 STT_NOTYPE  The symbol's type is not defined.

                 STT_OBJECT  The symbol is associated with a data object.

                 STT_FUNC    The symbol is associated with a function or other executable code.

                 STT_SECTION The symbol is associated with a section.  Symbol table entries of this type exist primarily for relo‐
                             cation and normally have STB_LOCAL bindings.

                 STT_FILE    By convention, the symbol's name gives the name of the source file associated with the  object  file.
                             A  file  symbol  has  STB_LOCAL  bindings,  its  section  index is SHN_ABS, and it precedes the other
                             STB_LOCAL symbols of the file, if it is present.

                 STT_LOPROC  This value up to and including STT_HIPROC is reserved for processor-specific semantics.

                 STT_HIPROC  This value down to and including STT_LOPROC is reserved for processor-specific semantics.

                 STB_LOCAL   Local symbols are not visible outside the object file containing their definition.  Local symbols  of
                             the same name may exist in multiple files without interfering with each other.

                 STB_GLOBAL  Global  symbols  are  visible  to all object files being combined.  One file's definition of a global
                             symbol will satisfy another file's undefined reference to the same symbol.

                 STB_WEAK    Weak symbols resemble global symbols, but their definitions have lower precedence.

                 STB_LOPROC  This value up to and including STB_HIPROC is reserved for processor-specific semantics.

                 STB_HIPROC  This value down to and including STB_LOPROC is reserved for processor-specific semantics.

                             There are macros for packing and unpacking the binding and type fields:

                             ELF32_ST_BIND(info) or ELF64_ST_BIND(info) extract a binding from an st_info value.

                             ELF32_ST_TYPE(info) or ELF64_ST_TYPE(info)
                             extract a type from an st_info value.

                             ELF32_ST_INFO(bind, type) or ELF64_ST_INFO(bind, type)
                             convert a binding and a type into an st_info value.

       st_other  This member defines the symbol visibility.

                 STV_DEFAULT     Default symbol visibility rules.
                 STV_INTERNAL    Processor-specific hidden class.
                 STV_HIDDEN      Symbol is unavailable in other modules.
                 STV_PROTECTED   Not preemptible, not exported.

                 There are macros for extracting the visibility type:

                 ELF32_ST_VISIBILITY(other) or ELF64_ST_VISIBILITY(other)

       st_shndx  Every symbol table entry is "defined" in relation to some section.  This member holds the relevant section header
                 table index.

       Relocation  is the process of connecting symbolic references with symbolic definitions.  Relocatable files must have infor‐
       mation that describes how to modify their section contents, thus allowing executable and shared object files  to  hold  the
       right information for a process's program image.  Relocation entries are these data.

       Relocation structures that do not need an addend:

           typedef struct {
               Elf32_Addr r_offset;
               uint32_t   r_info;
           } Elf32_Rel;

           typedef struct {
               Elf64_Addr r_offset;
               uint64_t   r_info;
           } Elf64_Rel;

       Relocation structures that need an addend:

           typedef struct {
               Elf32_Addr r_offset;
               uint32_t   r_info;
               int32_t    r_addend;
           } Elf32_Rela;

           typedef struct {
               Elf64_Addr r_offset;
               uint64_t   r_info;
               int64_t    r_addend;
           } Elf64_Rela;

       r_offset    This  member  gives the location at which to apply the relocation action.  For a relocatable file, the value is
                   the byte offset from the beginning of the section to the storage unit affected by the relocation.  For an  exe‐
                   cutable file or shared object, the value is the virtual address of the storage unit affected by the relocation.

       r_info      This member gives both the symbol table index with respect to which the relocation must be made and the type of
                   relocation to apply.  Relocation types are processor specific.  When the text refers to  a  relocation  entry's
                   relocation  type  or symbol table index, it means the result of applying ELF[32|64]_R_TYPE or ELF[32|64]_R_SYM,
                   respectively, to the entry's r_info member.

       r_addend    This member specifies a constant addend used to compute the value to be stored into the relocatable field.

       The .dynamic section contains a series of structures that hold relevant dynamic linking information.  The d_tag member con‐
       trols the interpretation of d_un.

           typedef struct {
               Elf32_Sword    d_tag;
               union {
                   Elf32_Word d_val;
                   Elf32_Addr d_ptr;
               } d_un;
           } Elf32_Dyn;
           extern Elf32_Dyn _DYNAMIC[];

           typedef struct {
               Elf64_Sxword    d_tag;
               union {
                   Elf64_Xword d_val;
                   Elf64_Addr  d_ptr;
               } d_un;
           } Elf64_Dyn;
           extern Elf64_Dyn _DYNAMIC[];

       d_tag     This member may have any of the following values:

                 DT_NULL     Marks end of dynamic section

                 DT_NEEDED   String table offset to name of a needed library

                 DT_PLTRELSZ Size in bytes of PLT relocs

                 DT_PLTGOT   Address of PLT and/or GOT

                 DT_HASH     Address of symbol hash table

                 DT_STRTAB   Address of string table

                 DT_SYMTAB   Address of symbol table

                 DT_RELA     Address of Rela relocs table

                 DT_RELASZ   Size in bytes of Rela table

                 DT_RELAENT  Size in bytes of a Rela table entry

                 DT_STRSZ    Size in bytes of string table

                 DT_SYMENT   Size in bytes of a symbol table entry

                 DT_INIT     Address of the initialization function

                 DT_FINI     Address of the termination function

                 DT_SONAME   String table offset to name of shared object

                 DT_RPATH    String table offset to library search path (deprecated)

                 DT_SYMBOLIC Alert linker to search this shared object before the executable for symbols

                 DT_REL      Address of Rel relocs table

                 DT_RELSZ    Size in bytes of Rel table

                 DT_RELENT   Size in bytes of a Rel table entry

                 DT_PLTREL   Type of reloc the PLT refers (Rela or Rel)

                 DT_DEBUG    Undefined use for debugging

                 DT_TEXTREL  Absence of this indicates no relocs should apply to a nonwritable segment

                 DT_JMPREL   Address of reloc entries solely for the PLT

                 DT_BIND_NOW Instruct dynamic linker to process all relocs before transferring control to the executable

                 DT_RUNPATH  String table offset to library search path

                 DT_LOPROC   Start of processor-specific semantics

                 DT_HIPROC   End of processor-specific semantics

       d_val     This member represents integer values with various interpretations.

       d_ptr     This  member  represents program virtual addresses.  When interpreting these addresses, the actual address should
                 be computed based on the original file value and memory base address.  Files do not contain relocation entries to
                 fixup these addresses.

       _DYNAMIC  Array  containing  all  the  dynamic  structures in the .dynamic section.  This is automatically populated by the
                 linker.

NOTES
       ELF first appeared in System V.  The ELF format is an adopted standard.

       The extensions for e_phnum, e_shnum and e_strndx respectively are Linux extensions.  Sun, BSD and AMD64 also support  them;
       for further information, look under SEE ALSO.

SEE ALSO
       as(1), gdb(1), ld(1), objdump(1), execve(2), core(5)

       Hewlett-Packard, Elf-64 Object File Format.

       Santa Cruz Operation, System V Application Binary Interface.

       UNIX System Laboratories, "Object Files", Executable and Linking Format (ELF).

       Sun Microsystems, Linker and Libraries Guide.

       AMD64 ABI Draft, System V Application Binary Interface AMD64 Architecture Processor Supplement.

COLOPHON
       This  page  is  part  of  release 3.54 of the Linux man-pages project.  A description of the project, and information about
       reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                                                       2013-04-17                                                      ELF(5)
